Polyethylene terephthalate is excellent in mechanical properties, electrical properties, heat resistance, chemical resistance and the like, and is used in various industrial products in the form of fibers or films. In the case of using polyethylene terephthalate as fibers or films, usually, they are subjected to stretching.
On the other hand, when polyethylene terephthalate is used for plastic articles, for example, as injection-molded products, it is known that many problems are present in molding, and further in respect of physical properties of molded products. In order to solve these problems, several techniques have been proposed.
For example, Japanese Patent Kokoku No. 44-457 discloses addition of glass fiber to improve heat distortion temperature. Japanese Patent Kokai No. 55-60553 discloses that crystallization sufficiently progresses even at a low mold temperature such as 80.degree. to 90.degree. C. by addition of a polyoxyalkylene compound having epoxy group and an elastomeric polyester having a glass transition temperature of not more than 10.degree. C. Further, Japanese Patent Kokoku No. 45-26225 discloses addition of a metal salt of a copolymer composed of .alpha.-olefin and .alpha.,.beta.-unsaturated carboxylic acid to a thermoplastic polyester to increase crystallization rate.
However, in the technique of the above Japanese Patent Kokoku No. 44-457, crystallization does not progress to the surface layer of a molded product, unless a high mold temperature such as 120.degree. to 140.degree. C. is employed. Therefore, a problem still remains in molding at a low mold temperature. In the technique of Japanese Patent Kokai No. 55-60553, crystallization progresses to the surface layer of a molded product even at a low mold temperature such as 80.degree. to 90.degree. C. and molding characteristics at a low temperature are considerably improved. However, usually, the maximum mold temperature is about 80.degree. C. because hot water is used to control the temperature of a mold, and therefore molding characteristics at a low temperature are still insufficient. Further, in the technique of Japanese Patent Kokobu No. 45-26225, although molding characteristics at a low mold temperature are improved, there is such a problem that mechanical properties such as tensile strength, tensile elongation at break, impact resistance and the like tend to lower.